Dimethylketene (DMK) has the formula: 
The existence of a carbon-oxygen and a carbon-carbon double bond allows one or the other of these double bonds to be opened selectively depending on the polymerization conditions. The polymerization of this monomer may therefore give rise to two different structural units: 
Regular head-to-tail linking of the units (A1) leads to polymers with a β-ketone structure and head-to-tail linking of the units (A2) leads to polymers with a polyvinylacetal-type structure. Alternating addition of the two units (A1) and (A2) leads to a polyvinyl ester structure: 
The polymer (A) used in the invention is such that the A2/(A1+A2) molar ratio is between 0 and 30%, meaning that the polymer (A) contains from 70 to 100 mol % of units (A1) and from 30 to 0 mol % of units (A2). It is then referred to as polydimethyl ketone, since the ketone units are predominant in the polymer.
In the packaging industry in general, and the food packaging industry in particular, it is very useful to have material which is insensitive to humidity, which is impermeable to water and which does not swell by absorbing water in a humid or aqueous medium. The reason for this is that the food packaging may accommodate beverages or foods containing a large proportion of water and may also undergo pasteurization- or sterilization-type treatments in the course of the packing process. Moreover, the impermeability of the packaging to water or to water vapour may be used to protect water-sensitive contents from a humid external atmosphere. This is particularly useful for the packaging of biscuitmaking, pastrymaking and bakery products which are sensitive to humidity, in the context of food packaging, and of anhydrous chemical products, in the context of industrial packaging.
Packaging to date has been produced using a multilayer polyolefin/tie/ethylene vinyl alcohol copolymer (EVOH)/tie/polyolefin structure. Within this structure the role of the EVOH is as a barrier to gases, in particular to oxygen, and the polyolefin layers (generally polyethylene or polypropylene) protect the EVOH from water, since EVOH losses its barrier properties in the presence of water or humidity. This structure presents the disadvantage of comprising a plurality of layers in order to be effective. It has now been found that it is possible to produce packaging using a monolayer structure of polydimethyl ketone type which has all of the properties required for use in an aqueous or humid medium: water-impermeable or a water barrier, insensitive to water (the physical and mechanical properties remain constant in the presence of water), a non-absorber of water and a barrier to oxygen whatever the humidity level of the medium.
U.S. Pat. No. 6,528,135, incorporated herein by reference, describes the gas, hydrocarbon and motor fuel barrier properties of polydimethyl ketone films. It also demonstrates that the oxygen barrier property of polydimethyl ketone is fairly insensitive to relative humidity, in contrast to that of EVOH. However, although this polymer is used for its stability in a hydrophobic medium such as hydrocarbons and motor fuels, there was nothing to suggest its qualities in the presence of humid or aqueous media.
The polydimethyl ketone of the invention is useful for a good number of applications which involve direct contact with a humid or aqueous medium. It is especially useful for manufacturing packaging which comprises at least one layer of polydimethyl ketone and which is intended for containing water or hydrophilic liquids such as for example beer, soda, fruit juice, milk and milk products, or foods which contain water by definition.